For decades Televisa’s logo – a golden human eye gazing at the world through a television screen – captured the company’s success at controlling and dominating what Mexicans watched.

The media firm, the biggest in Latin America, produced soap operas, quiz shows, films and news bulletins that reflected and reinforced the country’s concentration of economic and political power.

Televisa’s eye, with the pupil in the form of a globe, remained an unblinking stare during authoritarian one-party rule and Mexico’s transition to multiparty democracy, a change that only increased the company’s wealth and influence.

“You could say it’s like Murdoch on steroids in the sense Televisa has operated under far fewer constraints than Murdoch,” said Andrew Paxman, a historian and co-author of El Tigre, a biography of Emilio Azcárraga Milmo, the mogul whose father founded the company and the dynasty that still controls it.

Now, however, the logo suddenly evokes something else: a critical eye turned on Televisa itself in unprecedented scrutiny of the way it allegedly manipulates politicians and viewers.

The company’s alleged use – abuse, say critics – of programming for political and commercial ends has become an explosive issue in Sunday’s election. Student-led protesters have seized the agenda by marching on Televisa’s headquarters and calling the network a threat to democracy. The frontrunner for president, Enrique Peña Nieto, has been thrown on the defensive over evidence uncovered by the Guardian detailing his links to Televisa, whose channels account for about two thirds of free-to-air television. Its rival, Azteca, accounts for most of the other third.

“The power of the television networks does not lie in their economic power but in their ability to manipulate opinion. About 98% of homes have a television, and it is on between four to six hours a day in around 60% of homes,” said Purificación Carpinteyro, a former under-secretary of communications.

“The degree of concentration in television is an attack on democracy. It gives them enormous power to extort. The [networks] have the political class under control because nobody wants to be insulted or rubbed out or exhibited on TV. The television calling somebody corrupt is tantamount to a judgment from the supreme court.”

How Televisa acquired and uses its clout is a tale of intrigue worthy of the overwrought telenovelas – soaps – which it makes and exports across the Spanish-speaking world.

Emilio Azcárraga Vidaurreta started his empire with a Mexico City-based radio station in 1930. It grew into a chain of radio and television stations that in effect monopolised Mexico’s airwaves thanks to patronage from the Institutional Revolutionary Party (PRI), which ruled for most of the 20th century. Azcárraga returned the favour by turning news into PRI propaganda and slanting entertainment shows to reinforce its conservative ethos.

“It was a symbiotic relationship. Each side helped the other to retain its monopoly,” said Paxman, who teaches history at Millsaps College in Jackson, Mississippi. Telenovelas, for instance, preached knowing your place in society, of being happy with your lot.”

When Azcárraga Milmo died in 1997 his son, Emilio Azcárraga Jean, took over and transformed what had become a bloated behemoth into a slick media giant in time to adjust to the PRI’s loss of power in elections in 2000, ushering in a more complicated – and lucrative – multiparty era.

Generous state funds were given to parties to consolidate the fledgling democracy, making them big buyers of media advertising. They had additional funding for public awareness campaigns – de facto advertising – when running municipal, state and federal governments, making politicians the media’s dominant revenue source.

“Democracy is a good client,” Emilio Azcárraga Jean told the US-Mexican chamber of commerce in 2004. Televisa’s nebulous ideology facilitated a pragmatic shift to dealing not just with the PRI, which clung on to local and regional power bases, but with rivals such as the conservative National Action party (PAN) and the leftwing party of the Democratic Revolution (PRD).

“Before those with political power paid the media to be on their side, now the media charges. Paying for something is very different to being charged,” said Rubén Aguilar, a former spokesman for Vincente Fox, a PAN leader and president from 2000-06.

In 2007 the Mexican congress hit back at the media lords with a law giving political parties free TV advertising during elections and banning paid-for political spots. The networks responded to this threat to their income with fury, then subtlety. Santiago Creel, a senator who had championed the law, found himself erased from Televisa’s news reports. Seated in the middle of other legislators, all identifiable, he was transformed into a pixillated blur.

Azteca and Televisa also began getting associates elected to congress to defend their interests by adding them to the electoral lists of established parties. This year’s election is likely to yield a “telebancada” of between 10 and 20 deputies and senators.

After the new law Alejandro Quintero, Televisa’s vice-president of marketing, also allegedly intensified additional, creative ways to tap the political pot of gold by offering product placement packages to parties and candidates.

A character in the telenovela Un Gancho al Corazon, for instance, showed his support for the Green party on the eve of an election. The party was fined for veiled propaganda in an electoral period.

The season finale of the Mexican version of Ugly Betty, a highly successful soap, included a cameo with the governor of Monterrey and his wife. State institutions got in on the act: the federal security ministry, it was revealed, funded El Equipo, a prime time show in which charismatic police officers heroically fought drug traffickers. The series tag-line: “They know that good triumphs over evil.”

One national politician depicted the marketing executive as a Svengali who once said: “It’s a lie that we can invent a politician. Politicians need a long time to create an image. But what we can do is destroy them quickly.” Quintero declined to be interviewed for this article.

One part of promotional packages allegedly developed for Televisa by associated companies involves curbing criticism on news and other shows. “I used to be able to criticise anyone on my show – senators, ministers, the police, you name it,” said Héctor Suarez, a well known actor and comedian who left Televisa in November 2011 after 38 years. “But they started vetoing scripts. No you can’t do this, no you can’t say that. They shut me up.”

Much political promotion was subliminal, he said, and lulled Mexico into accepting manipulation: “An entertained country doesn’t conspire.” The student-led movement Soy132, which marched against Televisa, was an exception, said Suarez.

Most of Mexico’s traditional media – radio, newspapers and magazines – were just as mercenary as Televisa, said one editor, who declined to be named. “Ninety per cent of media income comes from public funds,” the editor said. Some journalists’ salaries were scaled according to willingness to disguise propaganda as reporting, he said.

A senior PRI official, he added, had recently offered his news organisation a large amount of cash for just one month’s positive coverage. Peña Nieto’s campaign team have denied the claims.Demetrio Sodi, a former PRD senator who once also stood for mayor for the PAN, denied personally paying for interviews but confirmed secretive deals were widespread. “What does happen is that a government that commits to buying a certain number of publicity spots will expect to have interview spaces opened for them.” TV’s power, he said, was not in news shows that few watched: “The real power are the entertainment shows.”

Enrique Peña Nieto, the PRI candidate and presidential frontrunner, married Angélica Rivera, a Televisa soap star who in adverts acted as the official face of the State of Mexico, of which he was governor.

Andrés Manuel López Obrador, a PRD leader and current presidential candidate, is widely believed to have made some kind of deal with Televisa to secure positive coverage while he was mayor of Mexico City.

He fended off scrutiny by sealing his budget records. Evidence uncovered by the Guardian suggests Televisa now champions Peña Nieto and tries to undermine his rival. The network declined interview requests for this article, saying it was still awaiting an apology from the Guardian for a previous article detailing its support for Peña Nieto, which it called libellous.